The present invention relates to a web handling system and, more particularly, to a system for incrementally moving webs of flexible, elastic sheet material to a processing station and precisely registering each web with the processing station.
In numerous applications, a web or continuous sheet of paper, film, plastic, or other material must be incrementally advanced to a processing station at which certain steps of manufacture or use are carried out with respect to the web while the web is stopped in registry with the processing station. Although it may be possible to perform these steps of manufacture or use upon the web while the web is actually moving through the processing station, many operations such as heat sealing, perforating, or cutting superimposed plastic webs require substantial time of engagement between sealing, perforating, or cutting apparatus and the webs relative to the high speeds at which it is economically desirable to advance the webs to the apparatus. To prevent one or more continuously advancing webs from being jammed and wrinkled or torn, the processing apparatus must be advanced with the web while it performs its function and then returned to an upstream portion of the advancing web for repeating its operation. This type of processing is often referred as "flying" and is known to be uneconomically expensive to implement and difficult to synchronize with the advancing web.
The difficulty of operating such flying apparatus is increased when a single processing apparatus is to operate on multiple webs because the advancing movement of each separately fed web must then also be synchronized with the other webs. Therefore, it is particularly desirable to incrementally advance separately fed webs to a processing station and then stop the webs in registry with the processing station during its operation.
Since webs are ordinarily obtained from a continuous supply such as a roll, the feeding of the web from the supply must be coordinated with the incremental stopping and starting of the web at the processing station. In a conventional installation, a pair of nip rollers engages the web adjacent the processing station. The rollers are driven to advance a desired length of web into the processing station and then stopped for the time necessary to complete the processing step in the station, the sequence being repeated for the length of the web. The incremental advance of the web presents a number of difficulties among which is the difficulty of accurately stopping and starting the unwinding of the web from a desirably large supply roll in increments and particularly short increments at which it may be desired to operate on the web in the processing station. This difficulty is accentuated when the web is both stretchable and fragile as, for example, thin webs of plastic used for making plastic bags which may be from 1.0 mil (about 0.003 cm.) to 10 mil (0.025 cm.) thick.
Presently known systems for such incremental web feeding suffer from several drawbacks which impose severe restrictions on their operation, particularly with respect to uniformity of increment lengths and speed. Conventionally, one pair of nip rollers is used to drive each of several webs to the processing station theoretically in like increments. Slippage or stretching of one web, however, will alter the feed-increment of one web relative to another. Upon stopping the roller, at least one web will then be misaligned with the processing station.
Independently feeding each of several webs to a common processing station also has been proposed as, for example, in U.S. Pat. No. 2,947,345. This arrangement is primarily intended to permit each web to be advanced at an independent rate but for substantially the same time so that different increments of each web may be fed to the processing station.
The patented arrangement further attempted to achieve more precise alignment of each web with the processing station by associating a photoelectric device with each web. The photoelectric devices detected marks placed along each web at increments corresponding to the increments at which the webs were to be aligned with the processing station. Upon detecting a mark, each photoelectric device sent a signal to a clutch-brake in the web-feeding means for that web to stop the web.
Such clutch-brakes, however, are mechanically incapable of the rapid, accurate starting and stopping necessary to produce precise registrations at economically desirable high web-feeding speeds. Backlash and other mechanical limitations of the apparatus as well as slippage and stretching of one web relative to another or even web breakage during its sudden stop makes impossible precise and rapid registration of each web with the processing station with such an arrangement.
A practical commercial apparatus which overcomes the problems associated with rapidly feeding a web and then indexing a portion of the web with a processing station, is disclosed in Bala U.S. application Ser. No. 481,918, now U.S. Pat. No. 3,948,425 filed June 21, 1974, and owned by the instant assignee. In the Bala apparatus, a variable speed servo-motor responds to an input voltage of preselected wave form to drive the nip rollers at speeds related to the instantaneous voltage amplitude. An encoder driven by the motor in conjunction with the web-feeding rollers generates pulses as the motor rotates so that the total number of pulses generated by the encoder is indicative of the length of the increment of web fed to the processing station.
A pulse counter counts the number of pulses generated by the encoder while the motor accelerates to and runs at full speed to rapidly feed most of the desired web-increment to the processing station. The counter then produces a signal which actuates control circuitry to cause the motor and thus the web to decelerate, the encoder generating a second group of pulses while the web decelerates. These latter pulses are used to develop a continuously decreasing analog voltage which in turn decelerates the motor. When the total number of pulses counted by the web control circuitry during the first web-accelerating and rapid-feeding stage and the second, deceleration stage reaches a preset number (which number corresponds to the length of increment of the web to be fed to the processing station), the motor, and the web, have come to a stop.
The apparatus of the Bala application is a significant advance over prior web handling devices, but while capable of feeding parallel webs simultaneously, it is not specifically adapted to maintain separate simultaneously fed webs in precise registry, where the webs are susceptible to misalignment because of stretching, supply variations, etc.